Transverse Electromagnetic (TEM) Resonator Coils
TEM resonator with 16 elements
Although birdcage coils provide excellent homogeneity at fields below 3T, at higher fields significant coupling between the patient and the coil may occur and dielectric effects become problematic. One solution that has become very popular at ultra high fields (and is now starting to appear in 3.0T scanners) is the transverse electromagnetic (TEM) resonator. A TEM resonator differs from a birdcage coil in the following ways: 1) the TEM coil typically uses foil microstrips instead of rods affixed to the inner surface of a nonconducting cylinder, and 2) on the outer surface of the cylinder is a slotted thin metallic shield.
The shield functions as an essential element of the system, providing a return path for currents from the inner conductor strips. Birdcage coils may have external shields, but these are separate and disconnected from the inner elements and used solely to reflect the fields inside the coil to prevent excessive radiation loss. Also unlike a birdcage coil, the TEM's inner conductors do not connect to their closest neighbors, but instead connect directly to the shield through tunable capacitive elements that can be adjusted to achieve the best homogeneity.
At high frequencies the TEM apparatus becomes a cavity resonator or longitudinal transmission line in which oscillating standing waves develop in accordance with known principles of antenna theory. At frequencies exceeding 200 MHz TEM coils produce more uniform RF-excitation and with higher quality factors than "standard" birdcage coils. With appropriate circuitry TEM resonators can be used as both RF-transmitters and receivers.
The shield functions as an essential element of the system, providing a return path for currents from the inner conductor strips. Birdcage coils may have external shields, but these are separate and disconnected from the inner elements and used solely to reflect the fields inside the coil to prevent excessive radiation loss. Also unlike a birdcage coil, the TEM's inner conductors do not connect to their closest neighbors, but instead connect directly to the shield through tunable capacitive elements that can be adjusted to achieve the best homogeneity.
At high frequencies the TEM apparatus becomes a cavity resonator or longitudinal transmission line in which oscillating standing waves develop in accordance with known principles of antenna theory. At frequencies exceeding 200 MHz TEM coils produce more uniform RF-excitation and with higher quality factors than "standard" birdcage coils. With appropriate circuitry TEM resonators can be used as both RF-transmitters and receivers.
References
Ibrahim TS, Leeb R, Baertlein BA, et al. Effect of RF coil excitation on field inhomogeneity at ultra high fields: A field optimized TEM resonator (pdf). Mag Reson Imaging 2001; 19:1339-1347.
Ivanov I,V Zubkov VI. Cavity Resonator. The Great Soviet Encyclopedia, 3rd Edition, 1979.
Vaughn JT. Radio frequency volume coils for imaging and spectroscopy (pdf). US Patent #5,886,596, March 23, 1999.
Vaughan JT, Adriany G, Garwood M, et al. Detunable transverse electromagnetic (TEM) volume coil for high-field NMR (pdf). Magn Reson Med 2002;47:990-1000.
Ibrahim TS, Leeb R, Baertlein BA, et al. Effect of RF coil excitation on field inhomogeneity at ultra high fields: A field optimized TEM resonator (pdf). Mag Reson Imaging 2001; 19:1339-1347.
Ivanov I,V Zubkov VI. Cavity Resonator. The Great Soviet Encyclopedia, 3rd Edition, 1979.
Vaughn JT. Radio frequency volume coils for imaging and spectroscopy (pdf). US Patent #5,886,596, March 23, 1999.
Vaughan JT, Adriany G, Garwood M, et al. Detunable transverse electromagnetic (TEM) volume coil for high-field NMR (pdf). Magn Reson Med 2002;47:990-1000.
Related Questions
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What is a birdcage coil and how does it work?
What are the function(s) of radiofrequency (RF) coils?